1. Field of the Invention
The present invention relates to magneto-optical recording systems, and more particularly to a process and an apparatus for information recording on a recording medium.
2. Description of the Prior Art
Magneto-optical recording system is a technical term derived from magneto-optical record-regenerating system, and may also be called photothermomagnetic recording system. In the magneto-optical recording system, the recording of binary information is typically achieved by an initializing step of aligning the magnetization of a magneto-optical recording medium, composed of a vertical magnetic layer such as GdCo or GdTbFe, in the upward or downward direction perpendicular to the layer face by means of a strong external magnetic field, and a step of forming a bit having vertical magnetization in the opposite direction. The bit formation is achieved by irradiating a recording track on the recording medium with a laser beam focused to a diameter of 1-2 microns to elevate the temperature of thus irradiated area close to the Curie point of the magnetic layer and thus to reduce the coercive force of said area to zero or almost zero, simultaneously applying a weak external bias magnetic field of opposite direction to invert the direction of magnetization, and terminating the irradiation with the laser beam whereby the irradiated area on the recording track is spontaneously cooled to fix the thus inverted direction of magnetization. In this manner there is formed a bit in which the direction of magnetization is inverted. Thus, if the original direction is taken as "0", the bit represents a signal "1", and the binary information is recorded by the presence and absence or length of the bit.
The binary information thus recorded can be read by irradiating the recording medium with a linearly polarized laser beam and detecting the direction of the plane of polarization in the reflected or transmitted light, which is rotated by the magnetic Kerr effect or Faraday effect. If the plane of polarization of the transmitted or reflected light is rotated by an angle .theta.k from the plane of polarization of the incident light when the magnetization is upward with respect to the direction of incident light, there will be obtained a rotation of -.theta.k when the magnetization is downward with respect to the direction of incident light. Consequently, if an analyzer is set in such a manner that the principal axis thereof is substantially perpendicular to a plane of angle -.theta.k, it does not transmit the light from an area of downward magnetization but transmits a portion, represented by sin.sup.2 2.theta.k, of the light from an area of upward magnetization. Thus, a photoelectric detector provided behind the analyzer generates electric information corresponding to the recorded magnetic information, in response to the scanning of the recording medium with the laser beam.
For re-using a recording medium which was already used in information recording, it is necessary to initialize the medium again in an initializing apparatus, or to provide a separate erasing head, or to erase the record with the recording head as a preliminary step prior to the information recording. However the initializing unit is bulky and expensive, and cannot therefore be attached to the recording apparatus. Also the use of a separate erasing head increases the manufacturing cost of the apparatus. Furthermore the preliminary erasure with the recording apparatus is not practical as it requires time as long as in recording. It is therefore highly desirable to erase the recorded information in a simple way simultaneously with the recording of new information. For achieving such simultaneous erasure and recording, it should be rendered possible to form a bit with a desired direction of magnetization regardless of whether the magnetization of the recording medium is upward or downward. For this purpose the bias magnetic field should be arbitrarily variable in direction, upward or downward, and, the speed of variation or modulating frequency should be in the order of mega Hz in order to attain a high recording speed. Without these features the magneto-optical record-regenerating system will not be more attractive compared with other recording-regenerating systems.
The bias magnetic field may be generated by a permanent magnet or an electromagnet, but only the latter is usable for modulating the magnetic field with a frequency in the order of mega Hz, since it is difficult to mechanically invert a permanent magnet with such a high frequency. However, even in the case of an electromagnet, a large current is required to apply a sufficiently strong magnetic field in non-contact state to the vertical magnetic layer of the recording medium, and it is difficult to modulate such a current with such a high frequency.
Consequently the bias magnetic field has had to be a fixed one, and simultaneous erasure-recording has been considered impossible in the magneto-optical recording system unless a separate erasing head is provided.
U.S. Pat. No. 4,472,748 discloses a method of simultaneous erasure and recording by inverting the direction of the bias magnetic field at every recording track and irradiating two positions on different recording tracks separated at least by another recording track therebetween respectively with an erasing light beam of a determined intensity and a recording light beam of a modulated intensity. This method, however, does not attain simultaneous erasure and recording on a given point on the recording track but merely effects erasure in a preceding track simultaneously with information recording in a succeeding track, and cannot be therefore called simultaneous erasure-recording in a strict sense.